What determines the size of the curl or wave in a permanent wave?

The size of the curl or wave in a permanent wave is primarily determined by the size of the rods used during the process. When smaller rods are employed, they create tighter, more defined curls, while larger rods produce looser waves or curls. This is because the hair wraps around the rod and conforms to its shape, and the diameter of the rod directly influences the final curl structure.

Other factors, while they do play a role in the overall outcome of a perm, do not specifically determine the curl size. For example, the type of hair product applied can affect the strength and longevity of the curl, but not the size. Similarly, the length of time the solution is applied impacts how well the hair is processed but does not influence the curl's size. The condition of the hair can affect how well the waves hold and how the hair reacts to the perm solution, yet it is the rod size that fundamentally decides the curls' dimensions.